Diurnal Variations in the Morphology of Schlemm's Canal and Intraocular Pressure in Healthy Chinese: An SS-OCT Study

Abstract

PURPOSE.To characterize the diurnal variations in the dimensions of the Schlemm's canal (SC) and its association with intraocular pressure (IOP) using swept-source optical coherence tomography (SS-OCT).METHODS. The temporal, nasal, inferior, and superior limbus of 102 eyes of 51 healthy subjects were imaged in vivo by SS-COT at 5 time points of 8 AM, 11 AM, 2 PM, 5 PM, and 8 PM. IOP was measured at the same time by Goldmann applanation tonometry (GAT). The diameter and the cross-sectional area of the SC were measured in ImageJ. The associations between changes in the SC parameters, IOP, and other biometric parameters were determined using a general estimating equations model. The temporal and inferior limbus of 94 eyes of 47 healthy subjects were also imaged before and during the Valsalva maneuver (VM) at 8 PM. RESULTS.Mean IOPs at different time points were 13.37, 12.89, 11.9, 12.02, and 12.36 mm Hg. Of all four quadrants, the detectable rate of SC was highest in the superior quadrant (85.3%) and lowest in the inferior quadrant (75.5%). We found that changes in the SC area and diameter were negatively associated with IOP changes only in the inferior quadrant (P ¼ 0.0046 and P ¼ 0.0332, respectively), after adjusting for age, sex, eye, spherical equivalent, and axial length. The mean SC area and diameter during the VM were significantly higher than prior to the VM (P < 0.001).CONCLUSIONS. The changes in the SC parameters were negatively associated with IOP changes only in the inferior quadrant. The VM could expand the SC in healthy subjects. Imaging of the SC may be a useful method to discover the reason why IOP fluctuates, and how SC changes morphologically during the daytime in the future.Keywords: Schlemm's canal, intraocular pressure, Valsalva maneuver T he rate of ocular aqueous humor (AH) drainage can affect intraocular pressure (IOP).1 In normal people, the aqueous humor is mainly drained through trabecular meshwork into the Schlemm's canal (SC), and then it flows into collector canals and episcleral veins. Aging, inflammation, and other immune factors could change the structural component in the outflow pathway of the AH, leading to imbalance in AH dynamics. [2][3][4] Blockage in any of the structures above will lead to the obstruction of AH drainage, causing ocular hypertension. 5Elevated IOP is an important risk factor for glaucoma and optic nerve damage. 6 Before the 21st century, the observation of SC in vivo was limited by imaging techniques. With the invention of highresolution optical coherence tomography (OCT), SC imaging in vivo became possible. Previous studies using OCT demonstrated that the SC area in patients with primary open angle glaucoma (POAG) and pseudoexfoliation glaucoma was decreased compared to normal people. [7][8][9] In addition, the morphology of SC can be affected by the application of prostaglandins and surgery. A study by Chen et al. 10 showed that the SC area expanded following the use of travoprost in healthy subjects. Similarly, expansion of SC was also detected after trab...

Age-Related Changes in Human Schlemm’s Canal: An in Vivo Optical Coherence Tomography-Based Study

“…In healthy subjects who suffered from acute intraocular pressure (IOP) elevation, as well as glaucoma eyes, decreased SC dimensions were reported ( Forgáčová et al, 2013 ; Hong et al, 2013 ; Kagemann et al, 2014b ; Xin et al, 2016 ). According to a recent research, only the CSA and the diameter of SC at the inferior quadrant were negatively associated with IOP changes ( Gao et al, 2017 ). Moreover, mutations of some genes, such as CYP1B1 and MYOC , were reported to be associated with the morphology of SC in congenital and juvenile glaucoma cases ( Choudhary et al, 2009 ; Hamanaka et al, 2017 ).…”

Aerobic exercise reduces intraocular pressure and expands Schlemm’s canal dimensions in healthy and primary open-angle glaucoma eyes

“…The cross-sectional area was drawn freehand twice by depicting the SC outline as described in our previous study. [ 27 ] The values of the SC parameters of superior, nasal, inferior, and temporal quadrants were included in the analysis. Two independent experienced technicians masked to the clinical data completed these measurements.…”

Association of sleep and sleep disorders with glaucoma

“…The pattern of variation in the IOP over the course of 24 hours may be related to other diurnal endogenous variations in the body, such as the production of cortisol. 28 The variations in IOP may be due to simple mechanical factors. One such mechanical factor is the tension in the intraocular muscles and the effect they have as they compress the globe during the contraction of ciliary muscles while accommodating.…”

“…The IOP is maintained at its normal level by balance between the hydrostatic pressure in the capillaries and the difference between the osmotic pressure of the aqueous humor and the capillary plasma. 28…”